The present invention generally relates to rotation control system in rotary recording medium reproducing apparatuses, and more particularly to a rotation control system which controls the rotation of the recording medium, so that the relative linear velocity between the rotary recording medium and a reproducing element for reproducing information from the rotary recording medium, is maintained constant regardless of the position of the reproducing element on the rotary recording medium.
Generally, recording is performed by rotating the rotary recording medium at a uniform rotational velocity, and upon reproduction, the rotary recording medium is rotated at a uniform rotational velocity. The above reproducing system is called the uniform angular velocity system (often abbreviated as the CAV system), since the rotary recording medium is rotated at an uniform angular velocity.
In the above CAV system, the relative linear velocity between the rotary recording medium and the reproducing element is reduced, when the position of the reproducing element on the rotary recording medium moves from the outer periphery of the rotary recording medium towards the inner periphery of the rotary recording medium. Accordingly, the worst S/N ratio is obtained at the innermost periphery of the rotary recording medium, since the relative linear velocity becomes the lowest at the innermost periphery of the rotary recording medium. When the number of rotation of the rotary recording medium is established so that an S/N ratio that can be put to practical use is obtained even at the innermost periphery of the rotary recording medium, the relative linear velocity at the outer periphery of the rotary recording medium becomes unnecessarily high, and hence, high recording density cannot be obtained. Furthermore, when only the outer peripheral parts of the rotary recording medium is used to avoid the use of the inner peripheral parts of the rotary recording medium wherein the S/N ratio is inferior, the recording capacity cannot be made large. For the above described reasons, difficulty is introduced especially when the diameter of the rotary recording medium which is to be used, is small.
In order to overcome the above described problems, a uniform linear velocity system (often abbreviated as the CLV system) has been proposed which rotated the rotary recording medium, so that the relative linear velocity between the rotary recording medium and the reproducing element is maintained constant regardless of the position of the reproducing element in the radial direction on the rotary recording medium. According to the CLV system, the above described problems are solved, since the relative linear velocity is always held constant even at the inner peripheral parts of the rotary recording medium.
The applicant has proposed a novel "Information signal recording and reproducing system" as disclosed in the specification of U.S. patent application Ser. No. 785,095, filed Apr. 6, 1977 (now U.S. Pat. No. 4,331,971), and in the specification of U.S. patent application Ser. No. 33,324, filed Apr. 25, 1979 (now U.S. Pat. No. 4,315,283), which is a Continuation-In-Part Application of the parent application Ser. No. 785,095. According to this system, the recording system forms pits in accordance with the information signal being recorded along a spiral track on a flat disc shaped recording medium, without forming a groove therein. In the reproducing system, a reproducing stylus traces over and along this track thereby to reproduce the recorded information signal in response to variations in the electrostatic capacitance.
By the use of this previous proposed system, the recording track has no groove. There is no possibility whatsoever of the reproducing stylus or the recording medium being damaged. The stylus can repeatedly trace the same portion of the track many times, where, by a so-called random access motion, a reproducing stylus is moved at high speed to a desired position on a disc to reproduce a desired information, in addition to a special reproduction such as still, slow motion, or quick motion reproduction.
In this system, however, since a groove for guiding the reproducing stylus is not provided on the disc, pilot or reference signals should be recorded on or in the vicinity of a track of the information signal, such as a video signal, on a rotary disc. At the time of reproducing, the reference signals are reproduced together with the video signal. Hence, the required tracking servo control is carried out so that the reproducing stylus accurately traces along the track in response to the reproduced reference signals. In this system, the reference signal for tracking control must be used.
Therefore, upon starting of a normal reproduction, the reference signal is reproduced as a signal having a frequency different from that of the actual frequency when the relative linear velocity between the rotary recording medium and the reproducing element is not equal to a predetermined value, and thus, normal tracking servo control is not performed. Accordingly, upon normal reproduction, the above relative linear velocity must be maintained at a predetermined velocity regardless of the position of the reproducing element, so that the reference signal is reproduced as a signal having the correct frequency.